Wear gloves when installing! This product is only worth the money paid for it at Sonic Electronix. I cannot say the difference in the sound quality is even noticeable. I installed Dynamat thoroughly in the front door inner steel panels on my wife's Focus. The back I did not. All 4 speakers installed are identical Alpines. One in each of the 4 doors.
I cannot tell the difference in sound quality. Alpine head unit driving all 4 speakers and no others.

The car as a whole seems a bit quieter and more "solid" with the Dynamat, but I seriously doubt the efficacy of the product for audio reproduction purposes.

If extraneous rattling/vibrating noises are your problem, duct tape and/or chimney roof flashing will be just as effective and cheaper.